The use of an electronic control and/or power-supply system on board a bicycle has already been proposed in the past, with particular reference to bicycles provided with electronically controlled motor-driven gear change. An example of solutions of this sort is described and illustrated in the U.S. Pat. No. 5,470,277 filed by the present applicant. For bicycles of the above type there exists the need to have available a housing for the electronic control unit that does not hinder normal use of the bicycle and at the same time enables access for possible replacement or for maintenance operations in an easy and fast way.